1. Field of the Invention
This invention relates to a heat exchanger. More particularly, this invention relates to a heat exchanger useful as an evaporator or a condenser mainly in an automobile air conditioner.
2. Description of Prior Art
The heat exchangers (evaporators) in the automobile air conditioners are mostly of the fin-and-tube type formed by combining round tubes and plate fins, the laminated type formed by laminating a plurality of tray-shaped plates thereby giving birth to tubes for the passage of coolant and nipping corrugated fins in the gaps between the adjacent tubes, or the profile tube type formed by extrusion molding a tube containing a multiplicity of through holes for passing coolant, zigzagging the tube and nipping corrugated fins between the adjacent folds of the zigzagged tube.
FIG. 1 is a cross sectional view illustrating a typical evaporator of the laminated type. This evaporator 1 is formed by laminating a multiplicity of tube units 5 each composed of a pair of tray-shaped plates (pieces) 2 joined to each other by the flanges 3 thereof after the fashion of the joined shell of a cream puff to enclose therewith a passage 4 for coolant and nipping corrugated fins 7 in the intervening spaces 6 between the adjacent tube units 5. The coolant which flows in via a conduit 8 on the inlet side, passes through the interiors of the tube 5, and flows out via a conduit 9 on the outlet side, therefore, exchanges heat with the air which flows along the fins 7.
A typical evaporator of the profile tube type has an appearance as shown in FIG. 2. This evaporator 11 is formed by zigzagging a flat tube 14 containing a multiplicity of through holes 13 for passing coolant and nipping corrugated fins 15 in the intervening spaces between the folds of the zigzagged tube 14. The coolant which flows in via a conduit 16 on the outlet side, passes through the interior of the tube 15, and flows out via a conduit 17 on the outlet side, therefore, exchanges heat with the air which flows along the fins 15.
FIG. 3 is a perspective view illustrating part of the evaporator of FIG. 2 in an enlarged state. In the corrugated fins 15, louvers 18 are formed as illustrated in FIG. 3. In the fins used in evaporators of other types, similar louvers are formed. The reason for the formation of the louvers 18 in the fins 15 in the manner described above is that the louvers 18 impart an edging effect to the fins 15 and, consequently, enhances the fins' overall heat-transfer efficiency without changing the total surface area of the fins and the louvers put together.
For any evaporator, the improvement of the efficiency of heat transfer between the coolant and the air constitutes an important question. Besides, the evaporator by nature collects water condensate on its surface from the moisture in the air. This condensate is desired to be removed from the evaporator as soon as possible without being scattered backwardly of the evaporator. The selection of a measure for this removal of the condensate, therefore, constitutes another important question.
The measure heretofore adopted for preventing the condensate formed on the evaporator from being scattered backwardly of the evaporator has consisted in disposing a metallic net behind the evaporator. The water condensate adhering to the fins of the evaporator, for example, may be blown backwardly of the evaporator when it is exposed to the air flowing along the fins and then backwardly of the evaporator. To preclude this mishap, the water condensate on the fins is desired to fall down inside the evaporator if circumstances permit.
From such a point of view, attention has been directed to the louvers formed in the fins and efforts have been made to utilize these louvers not merely for enhacing the efficiency of heat transfer but also for facilitating the dischage of water condensate.
To be more specific, when the air flowing along the fins incorporating these louvers comes into contact with the fins, there ensues deposition of water condensate on the fins. This water condensate has the possibility of being swept over the fins and then scattered backwardly of the fins by the current of air. To precluude this danger, the louvers have been adapted to guide the water and let it fall naturally under gravitation. To harness the gravitation for the purpose of enabling the water condensate to fall quickly without fail, the angle of inclination .theta. illustrated in FIG. 4 is desired to be increased as much as possible. Unfortunately, this angle cannot be increased amply because an increase of this angle .theta. could result in separation of the boundary layer formed on the surface of the louvers. The proposition that the desirable value of .theta. generally falls in the neighborhood of 15 degree has found recognition.
Various experiments have been conducted in search for the most desirable value of this angle of inclination .theta. of the louvers for the purpose of enabling the water condensate deposited on the fins to fall smoothly toward the lower part of the evaporator without the slightest sacrifice of the high efficiency of heat transfer. Consequently, the following knowledge has been acquired.
In the evaporator incorporating such conventional louvers as illustrated in FIG. 4, the main current F of air flowing between fins adjacent to each other in the vertical direction and the branched current f of air flowing between louvers adjacent to each other in the direction of the main current F have different flow speed; the flow speed of the branched current f is very small as compared with that of the main current F. Moreover, when the water condensate W adhering to the upper side of one louver is left growing in volume, it will bulge and eventually reach the lower side of the water condensate w similarly adhering to the immediately next louver to close the path for the branched current. It is possible that, depending on the relative location of the pair of louvers under discussion, the force with which the water condensate adheres to the louver under surface tension will surpass the sum of the force with which the water condensate is caused to fall under gravitation and the force exerted by the branch current. It is not inconceivable that when the water condensate grows to the extent of filling up the gap between the two adjacent louvers and consequently closes the path for the branched current f, the force with which the water condensate tends to remain in the gap will increase.
When the water condensate is left growing to the extent of closing the path for the branched current f, the surface tension of the water condensate will increase and, as the result, the water condensate subsequently collecting will be entrained by the passing air and carried backwardly of the evaporator instead of being caused to fall down under gravitation. This phenomenon may be precluded by increasing the intervals separating the louvers adjacent to each other in the direction of the main current of air. Unfortunately, such an addition to the intervals will result in a decrease in the number of louvers per unit length of the evaporator and in a sacrifice of the overall efficiency of heat transfer.
It is, therefore, an object of this invention to provide a novel heat exchanger.
Another object of this invention is to provide a heat exchanger so constructed that the heat transfer is improved to the highest possible extent where the intervals and length of the fins and the width and number of the louvers formed in the fins are fixed.
Yet another object of this invention is to provide a heat exchanger so constructed that when it is used as an evaporator, it will enable the water condensate adhering to the fins and their louvers to be quickly discharged.